In general, multilayer circuit boards are utilized in high power applications such as motor controllers, inverters, converters, power supplies, or other control devices. Typically, these boards include high-power electrical devices such as resistors and semiconductors which perform the functions required by the associated applications. As a result, these electrical devices often generate a significant amount of heat, and require heat sinks or other thermal management systems to prevent the circuit boards and electrical devices from overheating.
Heat sinks are typically metal components relatively large in size and secured to circuit boards or associated electrical devices to enhance heat dissipation therefrom. In particular, heat sinks are attached to a thermally and electrically conductive portion of an electrical device. For example, heat sinks are frequently secured directly to the lead frame of the device with hardware such as brackets, bolts, or other mountings. This additional hardware is expensive and increases the assembly time for the circuit board. The heat sinks are often electrically isolated from the lead frame with a heat conducting, electrically insulating layer of film or other material which is placed between the electrical device and the heat sink. Such a layer is disadvantageous because installing the layer increases the assembly time for the circuit board. Furthermore, the integrity of the layer is very difficult to inspect.
Some electrical devices are packaged as surface mount devices which utilize a minimum amount of space on the circuit board. However, higher power surface mount devices must be mounted on or near large pads or sections of the metal layer on the circuit board to provide adequate heat dissipation. These large sections are disadvantageous because the space required by the large sections could otherwise be utilized by other electrical components or eliminated to reduce the footprint of the board. To reduce the size of these large sections, surface mount devices are often mounted on circuit boards made from ceramic, aluminum-based substances, or other materials which have a high thermal capacity. Drawbacks with these types of circuit boards include their expense and weight.
Particular applications require circuit board systems which are optimized for superior heat dissipation. For example, in the field of electronic motor controllers, it is commonplace to build a controller package as an assemblage of circuit boards including a power substrate module or other heat dissipating medium. Each of the circuit boards supports components and conducting paths for accomplishing various functions in the completed device. Such motor controllers generally include control logic circuitry and power components. The control logic circuitry, typically including programmable solid state circuits such as a programmable logic controller mounted on a motherboard or a separate logic circuit module, monitors operating parameters of the motor and generates control signals for driving the motor in accordance with a preset control routine and various operator inputs. The power components typically include diode rectifying circuits for receiving AC power from a source and converting it to DC power, and power transistors or similar solid state switching devices, such as insulated gate bi-polar transistors (IGBTs), for converting the DC power to controlled AC signals for driving the motor based upon the control signals produced by the control circuitry. The power components are mounted on the power substrate module.
In motor controllers of this type, the board, substrate, or foundation for the power substrate module is often manufactured from an expensive ceramic or aluminum-based (e.g., Al.sub.2 O.sub.3) material having conductive lines and components on only a single side. This type of circuit board or substrate is expensive and increases the amount of space required for the motor controller package. In addition, due to the presence of different materials in such substrates, such as copper conductive layers, insulating layers, an aluminum-based heat dissipation layer and so on, high temperatures arising during operation of the power circuitry often lead to different amounts of thermal expansion between the various layers, resulting in considerable stress and even to failure of the substrate.
Another drawback of known power substrates arises from parasitic inductance between circuit components. Because power switching circuits are typically operated at a very high switching frequency, such inductance leads to voltage spikes, particularly in a turnoff phase of inverter operation. Such spikes are commonly reduced by the use of snubbing circuits, further adding to the cost and complexity of the substrate and supporting circuitry.
Thus, there is a need for a multilayer circuit board having an insulated mounting area for a surface mount device and a heat sink. There is also a need for a low cost multilayer circuit board optimized for heat dissipation and the reduction of parasitic inductance. There is further a need for a low cost circuit board which can be configured for use as a power substrate module in a motor controller.